Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

259
FXUS63 KLOT 291626
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1026 AM CST Sat Nov 29 2025

.KEY MESSAGES...

- A Winter Storm Warning remains in effect for the entire area
  through early Sunday morning.

- Periods of moderate to heavy snow will continue into this
  evening producing hazardous travel conditions. The highest
  snowfall rates and worst conditions are expected this
  afternoon into early this evening.

- Accumulating light snow (70-80% chance) late Monday afternoon
  into Monday night.

- Well below normal temperatures will persist through next week.

&&

.UPDATE...
Issued at 1026 AM CST Sat Nov 29 2025

The going forecast appears to be in pretty good shape. A
surface low continues to take shape across eastern Kansas and
will scoot northeastward into west-central Illinois this
afternoon and eventually right across our forecast area
overnight tonight. Ahead of the low, strengthening south to
southwesterly 700 mb flow will facilitate increasing warm
advection aloft through this morning and afternoon. At the same
time, a modest increase in upper jet divergence will also take
place as a westerly 110-120 kt jet streak begins to impinge upon
central/southern Illinois.

All of this will result in a continued gradual increase in
ascent, peaking this afternoon across the local area. Latest
forecast soundings continue to indicate that UVVs won`t be off
the charts by any means, however, with upward velocities peaking
generally in the 10-15 ubar/second range through a modestly-
deep dendritic growth zone north of about I-80. This should
yield a continued increase in snow quality and SLRs, with
hourly rates increasingly more solidly into the 0.75 to 1 inch
per hour range (briefly a locally perhaps a bit higher).

South of I-80, and particularly in the vicinity of about a
Gibson City to Rensselaer line, latest guidance depicts a local
enhancement to QPF amounts late this afternoon along the nose of
the the 850-700 mb LLJ. From about 2-6 PM, ascent is forecast
to maximize in this region as mid-level lapse rates steepen
ahead of the northward-advancing dryslot. If this level of
ascent is ultimately realized, a brief window of snowfall rates
of 1-2 inches per hour could be realized in this vicinity before
saturation is lost within the DGZ and surface temperatures
begin to warm above freezing. Have nudged amounts up just a hair
in this corridor, but this still remains a bit lower confidence.

Regarding total amounts: morning HREF guidance depicts widespread
coverage of 10-12 inch amounts in our area before all is said
and done. Given the general modest nature of most of the
parameter space, still a bit leery of this degree of coverage,
and continue to suspect that double digit totals should end up a
bit more isolated in our area. Other than a slight increase to
totals south of I-80, no other significant changes to the going
amounts appear necessary at this time.

Carlaw

&&

.DISCUSSION...
Issued at 226 AM CST Sat Nov 29 2025

Through Sunday Night:

No significant changes to the going forecast or reasoning.

Light snow continues to slowly spread into the area, saturating
the low levels. Once it begins, light snow will continue into
the morning hours and then increasing in intensity with periods
of moderate to heavy snow expected areawide through the
afternoon and into the early evening hours. Snowfall rates
during this time may approach 1 inch per hour. Southeast winds
will steadily increase through the morning hours with gusts into
the 30 mph range possible this afternoon. This will likely lead
to some blowing/drifting snow and expanded that mention across
the entire cwa. As the surface low pressure moves across the
northern part of the cwa this evening, warmer air will spread in
allowing temps to warm to near or just above freezing along and
southeast of I-55, allowing the snow to mix with or change to
some light rain or drizzle. By this time however, remaining qpf
amounts look rather light and while this may cut down on the
snowfall amounts, it may not make much difference for the snow
amounts. These warmer temps and possible mix should also end
the blowing/drifting snow potential.

As for qpf/snowfall amounts and snow ratios, no changes. Areas
southeast of a Pontiac to Valparaiso line will see the lowest
amounts, likely in the 5-9 inch range with areas northwest of an
Ottawa to Waukegan line seeing the highest amounts, with
perhaps a few areas reaching one foot across northwest IL.

After the low moves northeast of the area tonight, winds will be
shifting westerly, then northwesterly with cooler air spreading
back into the area. There will likely still be light snow or
snow showers falling overnight into Sunday morning and this snow
will be drier with some blowing/drifting snow possible again,
mainly across northwest IL. Though there may be a period of wind
gusts into the 30-35 mph range Sunday morning and blowing snow
may be possible wherever falling snow persists. It appears that
light snow will be ending from northwest to southeast across the
area Sunday morning with perhaps some flurries Sunday afternoon.

Lake effect snow may continue across Porter County Sunday
afternoon into early Sunday evening. Low confidence for how
long it may persist and for additional snowfall amounts and
while some headline extension may be needed for Porter County,
confidence is too low to make changes this morning. cms

Monday through Friday:
Issued at 255 PM CST Fri Nov 28 2025

The primary forecast focus beyond this weekend`s winter storm
is the increasing potential for another round of accumulating
snowfall during the Monday afternoon-Monday night. The multi-
model consensus this morning continues to advertise a secondary
shortwave dropping south through the Great Basin Sunday night
and eventually ejecting across the Central Plains on Monday as a
positively-tilted trough. This orientation would result in a
fairly progressive system overall, but intensifying mid-level
frontogenesis is forecast to result in an expanding region of
generally light snow across parts of our forecast area. A
significant north-south spread exists persists in the guidance
and ensemble output today, but in general, the favored location
for fgen-enhanced snowfall appears to be setting up somewhere
across the southeastern two-thirds of the forecast area.

Main concern during this period is that snow would be falling
into a cold airmass with surface temperatures in the upper teens
to mid 20s which would easily result in slick/hazardous travel
even with modest snowfall amounts. The other aspect that has our
attention is some degree of near-upright/convective instability
in recent model guidance above the main frontogenetic
circulations which could end up locally-enhancing precip rates.
At this time, liquid amounts are generally a quarter inch or
less which would support perhaps 2-4 inches of snowfall given
generally modest ascent through the DGZ and and cold surface
temperatures. Too much uncertainty at this point to pinpoint the
main threat area, but something we`ll be keeping a close eye on
over the coming days.

Increased cloud cover may end up tempering overnight lows/wind
chills a bit more than the currently-advertised NBM grids, but
cold conditions will nonetheless prevail next week. A
reinforcing shot of cold air is generally forecast to arrive
midweek as a roughly 1040 mb arctic high slides southward across
the central CONUS, perhaps with an additional round of snow or
at least flurries as the boundary layer saturates into the base
of a very deep dendritic growth zone.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 538 AM CST Sat Nov 29 2025

Aviation forecast concerns for the 12Z TAFs:

- Deteriorating cig/vis conditions this morning as snow
  gradually intensifies across the terminals. Periods of LIFR
  VSBYs are expected, particularly this afternoon and early
  evening (localized VLIFR possible).

- Snowfall rates will average around 0.5"/hr but will peak
  around 1"/hr this afternoon/early evening.

- Southeast winds become gusty near 25 kts by afternoon. May be
  a period of variable directions later this evening as surface
  low pressure center tracks across northern IL. Winds become
  gusty from the west-northwest late tonight/Sunday.

Well-advertised winter storm is taking shape across the region
early this morning, with surface low pressure along the KS/OK
border at 11Z. A broad area of light snow has already spread
into the forecast area predawn, and will gradually intensify
this morning as the system continues to lift northeast into the
region. Already seeing IFR vsbys at several of the TAF sites,
and expect vsbys to continue to gradually deteriorate to LIFR
through mid-late morning. A prolonged period of 1/2SM-3/4SM
vsbys appear likely this afternoon into early this evening in
periods of moderate snow, and some brief VLIFR can`t be ruled
out. Snow may mix with/change to drizzle later this evening,
possibly as far north as KMDW/KORD, but most likely at KGYY.
Snow intensity should taper off late this evening/overnight,
with gradual improvement to MVFR conditions expected by Sunday
morning though some light snow will likely persist.

As the pressure gradient tightens today ahead of the approaching
surface low, southeast winds will gradually increase with gusts
into the 25 kt range likely later this afternoon and early
evening. Most model guidance has the center of the low tracking
near/just north of KORD, which would support winds veering from
southeast to southwest around/after midnight, then becoming
west-northwest and becoming gusty again late tonight into
Sunday. The exact track of the low and details of the winds late
this evening are of somewhat low confidence prior to the shift
to west-northwest. Gusty winds may produce some blowing and
drifting of snow at times this afternoon/early evening as well
as on Sunday.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Warning until 6 AM CST Sunday for ILZ003-ILZ004-
     ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
     ILZ020-ILZ021-ILZ023-ILZ032-ILZ103-ILZ104-ILZ105-ILZ106-
     ILZ107-ILZ108.

     Winter Storm Warning until midnight CST tonight for ILZ033-
     ILZ039.

IN...Winter Storm Warning until 6 AM CST Sunday for INZ001-INZ002.

     Winter Storm Warning until midnight CST /1 AM EST/ tonight for
     INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 10 PM CST Sunday for the IL
     nearshore waters.

     Small Craft Advisory until 7 AM CST Monday for the IN
     nearshore waters.

&&

$$

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